1. Field of the Invention
The present invention relates to optical equipment provided with a lens position control device.
2. Related Background Art
There are already known various optical designs for an image-taking lens with variable focal length, a so-called zoom lens, employed for example in a video camera. Among such designs, most commonly known is a so-called 4-group zoom lens consisting of a 1st front lens group for focusing, a 2nd lens group for varying the image magnification, a 3rd lens group for correction and a 4th fixed lens group for image forming, wherein the 2nd and 3rd groups are linked in a predetermined relation to achieve the zooming operation. In such a 4-group zoom lens, the focusing by the 1st lens group is totally independent from the focal length adjustment by the 2nd and 3rd lens groups, so that it is not necessary to move the 1st lens group for zooming nor to move the 2nd lens group for focusing. For this reason a relatively simple mechanism can be employed in the lens barrel.
On the other hand, there is also known a zoom lens of the so-called inner focus type in which the focusing is achieved by the 3rd and following lens groups. In such as lens structure, different from the 4-group zoom lens mentioned above, the shortest focusable distance achieved by most advancing the focusing lens group is variable depending on the focal length. Particularly it provides an advantage, not achievable with the 4-group zoom lens, of focusing even to a very short distance just in front of the lens at the wide angle end. However, in such a zoom lens of inner focus type, the focusing lens group has to be moved in case of zooming even when the distance to the object is constant, since the focusing lens group is positioned behind the zooming lens groups. For this reason such a zoom lens requires a very complicated mechanism in the lens barrel, and has been little used in practice. Nevertheless, a recent development in the automatic focusing device has realized a system of directly evaluating the image blur on the focal plane and controlling the position of the focusing lens group based on thus-obtained information. Thus, the combination of an automatic focusing device and an inner focus type lens in such a system allows proper positioning the focusing lens group, without a complicated barrel structure.
FIGS. 5 to 8 illustrate certain examples of the inner focus lens. In a type shown in FIG. 5, a 1st lens group 1 is fixed. A 2nd lens group is movable between a solid-lined position 2 corresponding to the shortest focal length (wide angle end) to a chain-lined position 2' corresponding to the longest focal length (telephoto end). In this example a 3rd lens group is linked with the 2nd lens group with a certain relation as in the conventional 4-group zoom lens and is movable from a solid-lined position 3 (wide angle end) to a chain-lined position 3' (telephoto end). Said 2nd and 3rd lens groups are linked for example by a cam ring, as in the mechanism for the 4-group zoom lens. A focusing lens group 4 is rendered axially movable within a predetermined range as indicated by an arrow.
An example shown in FIG. 6 lacks the lens group 3 shown in FIG. 5. Also in this example, the lens group 4 is divided into a fixed front lens group 4A and a rear lens group 4B which is axially movable within a predetermined range for focusing.
In an example shown in FIG. 7, 1st and 4th lens groups 1, 4 are fixed, while a 2nd lens group is movable between a wide angle end position 2 and a telephoto end position 2'. A focusing lens group 3 is rendered axially movable within a predetermined range.
In an example shown in FIG. 8, a 1st lens group 1 is not fixed. The 1st and 2nd lens groups are moved in mutual linkage in zooming, between wide angle end positions 1, 2 and telephoto end positions 1', 2'. The focusing is achieved by a rearmost lens group 4B as in the example shown in FIG. 6.
FIGS. 9 and 10 show the relation, in the inner focus lenses shown in FIGS. 5 to 8, between the position of the focusing lens group and the focal length. FIG. 9 corresponds to the lenses shown in FIGS. 6 to 8 while FIG. 10 corresponds to the lens shown in FIG. 5, and the zero position in the ordinate indicates the position of the focusing lens group when focused to the infinite distance at the telephoto end.
In the lenses shown in FIGS. 6 to 8, as shown in FIG. 9, the shortest focusable distance is 0 m at the wide end, about 1 m at the intermediate position or about 0.6 m at the telephoto end. Also in the type of the lens shown in FIG. 5, said distance is 0 m at the wide angle end, and gradually increases to reach about 1 m at the telephoto end.
FIG. 11 shows the basic principle of an example of automatic focusing device of an the aforementioned type for directly evaluating the image blur on the focal plane. In FIG. 11A, there are shown an image frame 17 for example of a video camera, a distance measuring area for extracting a signal for automatic focusing, and a contrast pattern 19 of the object. FIG. 11B shows the method of signal processing. The contrast pattern (a) provides a luminance signal (b) which gives a differentiated signal (c). The absolute value of said signal is taken to obtain a signal (d), and sample holding provides a signal (e) with a height A. In a chart in FIG. 11 showing said value A in the ordinate as a function of the position of the focusing lens group in the abscissa, there is obtained a curve with a peak of which position (B) corresponds to the in-focus position of the focusing lens group.
FIG. 12 is a block diagram in case an inner focus lens shown in FIG. 6 is combined with such an automatic focusing device 12, 13. There are provided a sensor 12, an automatic focusing (AF) circuit 13 for detecting the focus state from the output of the sensor 12, and a motor 14 for axially driving a focusing lens group 4B.
In practice, however, in the structure shown in FIG. 12 it is often difficult to constantly maintain the in-focus state particularly during a zooming operation. This is because the track shown in FIGS. 9 and 10 for focusing to a given object distance cannot be traced due to the movement of the second zooming lens group in the course of the time required by the automatic focusing device 12, 13 to detect the image blur, to determine a front or rear focused state and to determine the direction of rotation of the motor 14.
In consideration of the foregoing, the present applicant already proposed, in the Japanese Patent Application Sho 63-109966, a method of dividing a map indicating the position of the focusing lens group in the ordinate as a function of the focal length in the abscissa, as shown in FIG. 9 or 10, into plural blocks I, II, . . . as shown in FIG. 13, then determining the moving direction and speed of the focusing lens group from the differentiated value of a track passing through the approximate center of each block and the moving speed of the second lens group in the course of the zooming operation, and activating the driving means for the zooming second lens group and that for the focusing lens group at the same time, even if the result of distance measurement is not obtained from the automatic focusing device, thereby preventing the out-of-focus state in the course of a zooming operation.
Also the inner focus lens explained above may become unable to be focused to an object if the focal length is improperly selected, since the shortest focusable distance at the most advanced position of the focusing lens group shown in FIGS. 9 and 10 varies depending on the focal length.
For avoiding such a drawback, the Japanese Laid-open Patent Sho 60-143310 discloses a method of determining, by calculation, whether the distance to the object identified by the automatic focusing device is shorter than the shortest focusable distance at the current focal length, and, if shorter, forcedly moving the zooming lens group toward the wider angle side.
Although such a method is relatively easily practiceable in case the in-focus lens position is predictable from the current position of the focusing lens group, as in the automatic focusing device of the so-called aberration detecting method, but is not applicable to an automatic focusing device capable only of identifying a rear or front focus state or incapable of detecting the exact distance to the object.
Another conceivable method for avoiding such a drawback consists, if an out-of-focus state is encountered at a short object distance, of moving the focusing lens group to the shortest distance position at the current focal length, and, if the out-of-focus state still continues, moving the zooming lens group toward the wider angle side.
However, this method will require a longer time before the in-focus state is reached.